Viruses, Viroids, and Prions

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Viruses, Viroids, and Prions
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Are Viruses Living or Non-living?

• Viruses are both and neither
• They have some properties of life but not others
• For example, viruses can be killed, even
  crystallized like table salt
• However, they cant maintain a constant internal
  state (homeostasis).

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What are Viruses?

• A virus is a non-cellular particle made up of
  genetic material and protein that can invade
  living cells.

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Viral History
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Discovery of Viruses

• Beijerinck (1897) coined the Latin name virus
  meaning poison
• He studied filtered plant juices found they
  caused healthy plants to become sick

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Tobacco Mosaic Virus

• Wendell Stanley (1935) crystallized sap from sick
  tobacco plants
• He discovered viruses were made of nucleic acid
  and protein

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Smallpox

• Edward Jenner (1796) developed a smallpox vaccine
  using milder cowpox viruses
• Deadly viruses are said to be virulent
• Smallpox has been eradicated in the world today

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Viewing Viruses

• Viruses are smaller than the smallest cell
• Measured in nanometers
• Viruses couldnt be seen until the electron
  microscope was invented in the 20th century

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Size of Viruses
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Viral Structure
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Characteristics

• Non living structures
• Noncellular
• Contain a protein coat called the capsid
• Have a nucleic acid core containing DNA or RNA
• Capable of reproducing only when inside a HOST
  cell

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Characteristics
CAPSID

• Some viruses are enclosed in an protective
  envelope
• Some viruses may have spikes to help attach to
  the host cell
• Most viruses infect only SPECIFIC host cells

DNA
ENVELOPE
SPIKES
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Characteristics

• Viral capsids (coats) are made of individual
  protein subunits
• Individual subunits are called capsomeres

CAPSOMERES
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Characteristics

• Outside of host cells, viruses are inactive
• Lack ribosomes and enzymes needed for metabolism
• Use the raw materials and enzymes of the host
  cell to be able to reproduce

EBOLA VIRUS
HIV VIRUS
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Characteristics

• Some viruses cause disease
• Smallpox, measles, mononucleosis, influenza,
  colds, warts, AIDS, Ebola
• Some viruses may cause some cancers like leukemia
• Virus-free cells are rare

MEASLES
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Viral Shapes

• Viruses come in a variety of shapes
• Some may be helical shape like the Ebola virus
• Some may be polyhedral shapes like the influenza
  virus
• Others have more complex shapes like
  bacteriophages

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Helical Viruses
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Polyhedral Viruses
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Complex Viruses
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Taxonomy of Viruses
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Viral Taxonomy

• Family names end in -viridae
• Genus names end in -virus
• Viral species A group of viruses sharing the
  same genetic information and ecological niche
  (host).
• Common names are used for species
• Subspecies are designated by a number

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Viral Taxonomy Examples

• Herpesviridae
• Herpesvirus
• Human herpes virus 1, HHV 2, HHV 3
• Retroviridae
• Lentivirus
• Human Immunodeficiency Virus 1, HIV 2

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Herpes Virus
SIMPLEX I and II
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Adenovirus
COMMON COLD
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Influenza Virus
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Chickenpox Virus
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Papillomavirus Warts!
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Used for Virus Identification

• RNA or DNA Virus
• Do or do NOT have an envelope
• Capsid shape
• HOST they infect

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Bacteriophages
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Phages

• Viruses that attack bacteria are called
  bacteriophage or just phage
• T-phages are a specific class of bacteriophages
  with icosahedral heads, double-stranded DNA, and
  tails

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T-phages

• The most commonly studied T-phages are T4 and T7
• They infect E. coli , an intestinal bacteria
• Six small spikes at the base of a contractile
  tail are used to attach to the host cell
• Inject viral DNA into cell

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Escherichia Coli Bacterium
T - EVEN PHAGES ATTACK THIS BACTERIUM
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T-Even Bacteriophages
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Diagram of T-4 Bacteriophage

• Head with 20 triangular surfaces
• Capsid contains DNA
• Head tail fibers made of protein

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Retroviruses
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Characteristics of Retroviruses

• Contain RNA, not DNA
• Family Retroviridae
• Contain enzyme called Reverse Transcriptase
• When a retrovirus infects a cell, it injects its
  RNA and reverse transcriptase enzyme into the
  cytoplasm of that cell

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ENZYME
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Retroviruses

• The enzyme reverse transcriptase (or RTase),
  which causes synthesis of a complementary DNA
  molecule (cDNA) using virus RNA as a template

RTase
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Retroviruses

• HIV, the AIDS virus, is a retrovirus
• Feline Leukemia Virus is also a retrovirus

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Viroids Prions
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Viroids

• Small, circular RNA molecules without a protein
  coat
• Infect plants
• Potato famine in Ireland
• Resemble introns cut out of eukaryotic

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Prions

• Prions are infectious proteins
• They are normal body proteins that get converted
  into an alternate configuration by contact with
  other prion proteins
• They have no DNA or RNA
• The main protein involved in human and mammalian
  prion diseases is called PrP

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Prion Diseases

• Prions form insoluble deposits in the brain
• Causes neurons to rapidly degeneration.
• Mad cow disease (bovine spongiform encephalitis
  BSE) is an example
• People in New Guinea used to suffer from kuru,
  which they got from eating the brains of their
  enemies

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Viral Replication
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Viral Attack

• Viruses are very specific as to which species
  they attack
• HOST specific
• Humans rarely share viral diseases with other
  animals
• Eukaryotic viruses usually have protective
  envelopes made from the host cell membrane

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5 Steps of Lytic Cycle

• 1. Attachment to the cell
• 2. Penetration (injection) of viral DNA or RNA
• 3. Replication (Biosynthesis) of new viral
  proteins and nucleic acids
• 4. Assembly (Maturation) of the new viruses
• 5. Release of the new viruses into the
  environment (cell lyses)

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Bacteriophage Replication

• Bacteriophage inject their nucleic acid
• They lyse (break open) the bacterial cell when
  replication is finished

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Lytic Cycle Review

• Attachment Phage attaches by tail fibers to
  host cell
• Penetration Phage lysozyme opens cell wall,
  tail sheath contracts to force tail
  core and DNA into cell
• Biosynthesis Production of phage DNA and
  proteins
• Maturation Assembly of phage particles
• Release Phage lysozyme breaks cell wall

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Bacterial cell wall
Bacterial chromosome
Capsid
DNA
Capsid
Sheath
Tail fiber
Tail
1
AttachmentPhage attaches to host cell.
Base plate
Pin
Cell wall
Plasma membrane
2
PenetrationPhage pnetrates host cell and
injects its DNA.
Sheath contracted
Tail core
3
Merozoites released into bloodsteam from liver
may infect new red blood cells
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Tail
DNA
4
MaturationViral components are assembled into
virions.
Capsid
5
ReleaseHost cell lyses and new virions are
released.
Tail fibers
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One-step Growth Curve
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Viral Latency

• Some viruses have the ability to become dormant
  inside the cell
• Called latent viruses
• They may remain inactive for long periods of time
  (years)
• Later, they activate to produce new viruses in
  response to some external signal
• HIV and Herpes viruses are examples

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Lysogenic Cycle

• Phage DNA injected into host cell
• Viral DNA joins host DNA forming a prophage
• When an activation signal occurs, the phage DNA
  starts replicating

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Lysogenic Cycle

• Viral DNA (part of prophage) may stay inactive in
  host cell for long periods of time
• Replicated during each binary fission
• Over time, many cells form containing the
  prophages

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Viral Latency

• Once a prophage cell is activated, host cell
  enters the lytic cell
• New viruses form a the cell lyses (bursts)
• Virus said to be virulent (deadly)

ACTIVESTAGE
INACTIVE STAGE
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Virulent Viruses
HOST CELL LYSES DIES
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The Lysogenic Cycle
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Latency in Eukaryotes

• Some eukaryotic viruses remain dormant for many
  years in the nervous system tissues
• Chickenpox (caused by the virus Varicella
  zoster) is a childhood infection
• It can reappear later in life as shingles, a
  painful itching rash limited to small areas of
  the body

SHINGLES
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Latency in Eukaryotes

• Herpes viruses also become latent in the nervous
  system
• A herpes infection lasts for a persons lifetime
• Genital herpes (Herpes Simplex 2)
• Cold sores or fever blisters (Herpes Simplex1)

SKIN TO SKIN CONTACT
PASSED AT BIRTH TO BABY
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Virulence

• VIRUS DESTROYING HOST CELL

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Lytic and Lysogenic Cycles
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Treatment for Viral Disease
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Vaccines

• An attenuated virus is a weakened, less vigorous
  virus
• Attenuate" refers to procedures that weaken an
  agent of disease (heating)
• A vaccine against a viral disease can be made
  from an attenuated, less virulent strain of the
  virus
• Attenuated virus is capable of stimulating an
  immune response and creating immunity, but not
  causing illness

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Other Viral Treatments

• Interferon are naturally occurring proteins made
  by cells to fight viruses
• Genetic altering of viruses (attenuated viruses)
• Antiviral drugs (AZT)
• Protease inhibitors prevent capsid formation